HARDNESS TESTING AND TYPES

                           HARDNESS TESTING.

                        The hardness test is mechanical test which used to check the hardness of material, Resistance of a material to permanent deformation as indentation wear etc called hardness.The hardness of material depends upon tensile/yield strength,grain size etc.

                  Currently many nos. of testings used for hardness checking some of this listed below,

1. Rockwell Hardness.
2. Brinell Hardness.
3.  Vickers hardness.
4. Shore schlerescope.

          In most of hardness checking tastings, impressing diamond or ball on a surface of material which is to be checked, take readings and resultant hardness values given by machine and the readings are in BHN or HV depends on test which is to be choose. Generally Hardness checked on weld area also on heat affected zone  (HAZ) checking the value and resultant value must be lower that the client requirement if exceeds then rectification or refabrication to be done.

                  In a pressure vessels generally after PWHT (Heat Treatment) the hardness testing carried out by the technician. The requirement of hardness checking on WELD or HAZ area inside the vessel or outside of vessels depends on client requirements. The hardness checking tester calibrated by standard calibration block before checking of hardness and the block should be calibrated in specific interval of time.Calibration certificate is required for review.

• Brinell Hardness -  

                                 This type of hardness checking for soften material like mind steel, low carbon steel, aluminium or copper etc. The main principle of this type of hardness based on area of penetration area. In this type of hardness testing the 3000 kg is applied on indental ball of  (10 mm dia) for 10 seconds.

                 Brinell hardness calculated by formula,

• Rockwell Hardness - 

                                    This hardness technique is most widely used due to easiness similar to Brinell technique. In this type of hardness technique the indentation tool used made up of diamond material,shape of the ball is conical.The Rockwell hardness check for all materials except some very hard material. The main principle of this technique is based on depth of penetration of ball.

                         

                               One thing is noted that the Rockwell hardness value is inversely proportional to depth of penetration as the more depth of penetration of ball,hardness value is less and vice-versa. 

• Vickers Hardness - 

                                  This technique is similar to Brinell test as the tool material used for indentor is made up of diamond. The shape of indentor tool is pyramid with base is in square shape and angle between apposite edged is 136 degrees. The main principle of this technique is diagonals of indentor is left on the surface of material. Vickers hardness calculated by following formula,

                             
                     


         For hardness checking of high carbon steel or tungsten carbide this technique is used.

• Shore schlerescope - 

                                    In this technique the diamond ended hammer is falls inside the glass tube  under the specific amount of force (own weight of hammer also)from fix specific height on the material,when the hammer is rebound the height of rebounded hammer is measured on graduate scale, this scale is consist of 100 parts, this scale may increase when the material having high hardness.

                                The value of schlerescope hardness is depends upon the rebound of hammer so as a material is harden then more the hammer rebound and the height of rebounded hammer is more so that readings is taken.Main advantage of this type of technique is portable.

                              If workman is trained for operation of this hardness then this hardness checking technique is easy.

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